This invention relates to an arrangement for protecting sensitive electronic equipment from the effects of transient voltage surges which may appear on AC power lines.
It has long been known that electronic equipment can be damaged, or even destroyed, by transient voltage surges which appear on the AC power lines to which the equipment is connected. These surges can be caused by, for example, a motor turning off. However, the most powerful of these surges are caused by lightning. Studies have shown that lightning can cause voltage spikes of up to 6,000 volts to appear on residential power lines. It is therefore a primary object of this invention to provide an arrangement which will afford protection against such transient occurrences.
To provide such protection, the transient voltage must be clamped to a safe level and the current must be dissipated. Most surge suppression units presently on the market use only a metal oxide varistor (MOV). The MOV clamping voltage must be above the peak of the highest expected line voltage. For nominal 120 volts AC, with a +10% variation tolerance this means that the line voltage could be 132 volts AC. The peak voltage would be this times 1.414, or 186 volts. Allowing for a 20% tolerance variation of the MOV, this brings the MOV rated clamping voltage to 225 volts. A one inch diameter MOV, rated at 70 Joules, can absorb as much as as 6,500 amperes of surge current before being destroyed. However, the high currents, along with the internal resistance of the MOV, allows the clamping voltage level to exceed its rating, which is usually at 10 amperes. Real clamping levels at 3,000 amperes can exceed 400 volts. The 400 volts clamping level is usually enough to damage most electronic equipment.
Another commonly used component is the AC zener diode, which is really a pair of back-to-back zener diodes. This device has the advantage of a 5% tolerance and a lower impedance than the MOV. Accordingly, a lower clamping voltage can be specified without fear of coming too close to the 186 volts line peak. A 195 volts 5% diode is usually specified. However, the current handling capability of the diode is limited to several hundred amperes and a 3,000 amp transient would certainly destroy it. Even so, it does have the advantage of maintaining its clamping voltage throughout its current range.
Another commonly used component is the gas discharge tube. Such a device operates very slowly and ignores fast rise time transients. It also has the disturbing characteristic of remaining shorted until all current has been removed. Thus, it typically turns a transient surge into a total loss of power and a tripped circuit breaker.
Many manufacturers stack different types of clamping components across the power line in the hope that if one type fails to clamp then the other type might take over clamping. Unfortunately, the device with the lowest clamping voltage will conduct all the current while the higher voltage devices will sit idle unless the lower voltage device blows out. In fact, some surge protection units have low amperage fuses to do just that. However, this does not protect equipment which is still connected to a high frequency 400 volts transient.
Still other units are provided with an indicator between clamping stages. Some of these inductors are connected to a capacitor to provide filtering while others act to delay the transient surge between stages.
However, all of the previously known arrangements have not proven to be entirely satisfactory. It is therefore an object of this invention to provide an improved transient voltage surge suppressor which is devoid of the drawbacks of previously known arrangements.